1. Field of the Invention
The present invention relates to a process for producing a ferromagnetic metal powder.
2. Description of Prior Arts
In recent years there have been extensively studied and developed magnetic recording media such as magnetic tapes using a ferromagnetic metal powder which is a magnetic substance having a high coercive force (Hc) and high saturation magnetization (.sigma.s) for the purpose of improving the recording density and reproducing output.
As one of processes for producing a ferromagnetic metal powder for use in magnetic recording media, there is known a process wherein acicular iron oxyhydroxide or an acicular metal oxyhydroxide composed mainly of iron is dehydrated by heating in a non-reducing atmosphere to produce iron oxide particles or metal oxide particles and the resulting iron oxide or metal oxide particles are then reduced by heating in a reducing atmosphere.
It is known that the above-stated process for producing the ferromagnetic metal powder has disadvantages, particularly a problem in that since the reduction in the above process is carried out by heating in a hydrogen stream atmosphere, the powder is liable to sinter in the course of the reduction. It is known that the coercive force of the metal powder depends mainly on anisotropy in shape defined by the acicular nature of particles so that it is important to retain the acicular nature of the material.
In order to avoid sintering in the course of the reduction by heating, heretofore, there has been employed a process wherein the starting material (i.e., acicular oxyhydroxide) is previously treated with a compound having an effect of inhibition of sintering (i.e., sintering inhibitor) to have the sintering inhibitor deposited or adsorbed on the surface of the material, and the treated oxyhydroxide is dehydrated by heating in a non-reducing atmosphere and then reduced by heating in a reducing atmosphere. As the sintering inhibitor, silicon compounds are disclosed in, for example, Japanese Patent Provisional Publication Nos. 52(1977)-134858, 56(1981)-156706 and 57(1982)-63605.